Use the GTD tab method to organize your Moleskine. Quite simply you use self-adhesive tabs, such as Avery Write-On Tabs or 3M Post-it "Durable Index Tabs" to separate your Moleskine into sections. You then label those sections in a way that helps you implement the GTD system. For example, you can divide it into the three sections: next actions, projects list, and someday maybe list. You can then write from the back cover in and use that as your Inbox to capture random ideas on the fly. Alternatively you can use the colored tabs to represent contexts or projects.

The Avery tabs are great because they are removable and it is therefore easy to make the size of sections bigger or smaller. Implementing this hack on a pocket Moleskine is great because you can then have your Inbox and entire GTD system with you all the time, wherever you go, in your back pocket. Add a Cross Ion or an Inka pen on your keychain, and it's a complete Hipster PDA solution.

If you are worried about tabs sticking out or being damaged in cluttered bags, and you don't need to see a text label to find a division, try Page Points or BookDarts. They're great for flipping right to a page, and they look pretty too. They are easy to move around and you can store a few extras on the back page or the accordion for marking a particular thought.
Moleskine Travel Info
Another Moleskine-GTD-Hack: As of 2007, you can buy books, which have five tabbed sections. Just use your labeler to relabel these sections for implementing GTD.

I accidentally discovered that a thick guitar pick makes an excellent, and usefull, place marker for Moleskine Reporter Notebooks.

I cut very small tabs out of the corner of my mini-reporters notebook, one tab for each GTD context.

I don't have enough appointments to justify carrying a separate planner in addition to my large squared notebook, so I turned my last twelve pages in to monthly planner.

Obtain a simple yearly calendar. The output of the Unix cal command is excellent for this purpose.

Cut out each month.

Fasten a single month to each page with rubber cement.

I write the date of the appointment with its description in the approximate position on the page that corresponds to the day's position in the month (e.g., an appointment for 15 July would be about halfway down the page.)

For you GTD aficionados who already own a labelmaker, take a moment or two to stick a label on the front cover of each notebook so you can tell them apart. Mine get my initials and the date of the first entry.

If you use a Cahier, with a soft cover, and click your spacepen inside, you'll soon wear away a portion of the cover. To prevent this, clip one bookdart to the cover where the pen clip rides (cost: approx 7 cents). When you retire the Moleskine from active use, move the bookdart to the new notebook.

Placing a calendar inside the back cover often comes in handy. There are many free calendar generators on the web. My Free Calendar Maker can generate a calendar page, but plenty of others would work just as well.
It may also be useful to quickly generate a calendar using the UNIX cal command. Printed out, they perfectly fit into the back cover or any other page. Use them to mark when to finish certain tasks, etc.

Remember those sticky-back vinyl pockets made for keeping 3.5" disks in your file folder? Stick one of those on the inside cover of your large-size Moleskine, and you have a handy pocket for your Levenger note cards (or whatever you keep your to-do lists on).

If you are carrying 3x5 cards around, you will quickly note that the Moleskine pocket can only hold a few of them. Fix that by reversing the accordion folds in the pocket (a small amount of ungluing is all that's required) which makes the pocket much bigger.

I know some of you, like me, are multiple-Moleskine nerds. Itâ€™s sad, but this is how Godâ€™s made us. So, this means you might have a lined notebook, a sketch book, a music notebook, or even a storyboard notebookâ€”all of the same size and outward appearance. Using a silver Sharpie, correction fluid pen or the like, make a small icon or letter at the same place on each spine to remind you which is which.

In the same manner as the spine icons, you can use a silver Sharpie, or something similar, to give your Moleskine a title or summarize the contents. This can be done before or after your have filled the pages. For example, you may want to label your Moleskine journal by the month and year you began writing in it and the same for when you finished; Journal: Dec. 2003 - May 2004. You may also like to label a collection of sketches or stories into one volume and use the Sharpie to indicate it as such; Watercolors 2004.

I carry three or four Post-It notes on the inside front cover of my pocket-sized Moleskine, and when I need to jot down a few quick temporary notes I slap one onto the outside front. Adhesive edge towards the spine, so the elastic band keeps the loose edge from flapping too much and yanking the note off in my pocket.

Note that Post-It adhesive leaves a residue and is also acidic. If your Moleskine is for daily use, you may not care, but if you are using it as a diary or long term sketchbook, you want to avoid the use of Post-It or similar temporary adhesive products. Reference: http://orpheus.ucsd.edu/preservation/postits.html

A very simple formula for my Moleskine right now: Moleskine (lined) + Categorical flags at sensible points inside + index cards + ThinkBlue2008 band = coolness. The moleskine with its flagged sections is for getting all the thoughts down on paper relating to the general content divisions of my life (Personal, Political, Media, and Writing, right now). I use the Index Cards to collate context-specific things I have to do, using the unsorted moleskine pages as a base. They're also just good to have, really. I like the sanctity of the Moleskine and always hate tearing out pages. Lastly, I have the ThinkBlue band to act as a horizontal binder of index cards to moleskine. It works quite well, I think.

The advantage that it has over just storing index cards in the accordion is basically just visibility: my context specific lists are always visible, no opening of notebook/flipping of pages necessary. Along with this is access: if I need to quickly write an addition to the currently applicable context-specific card (perhaps while driving? shh...) then it's not a difficult maneuver.

Although I still swear by my Fisher Space Pen when Iâ€™m on the road, Iâ€™ve become a recent convert to the Pilot G2 gel pen. It feels great on a Moleskineâ€™s silky pages. Nice sharp line thatâ€™s more even than the Space Penâ€™s, I must admit. Sits well in your hand, too. But if you want to use a real plume, you might want to look at: Moleskine Friendly Fountain Pens